Jiang Yanyong, a former military surgeon, has not chosen to spend his retirement quietly.

He shot to public prominence in April 2003 for revealing the Chinese authorities were covering up the extent of the Sars epidemic in Beijing.

Now, aged 72, he has taken on the leadership again, calling on the party to reappraise the events of June 1989, when hundreds of people died in democracy protests brutally suppressed by the authorities.

He has sent a letter to the party leaders, asking them to address what he calls a "mistake".

I felt I had to reveal what was happening, not just to save China, but to save the world

Jiang Yanyong on Sars

Dr Jiang said that on the night of 4 June, he saw 89 patients with bullet wounds in his emergency ward in a two-hour period.

"The vast majority of people I know in every quarter of society are all clear in their hearts that the June 4 crackdown was absolutely wrong," Dr Jiang wrote. "But because of the pressure from above, they haven't dared to speak their mind."

Dr Jiang has spoken out, even though his plea is a dangerous one and is unlikely to be heeded by the Communist Party.

"I have considered the possibility that by writing this letter I will encounter various consequences, but I nevertheless decided to explain my exact views to you," he wrote.

It was this same urge to act that led him to write to the Chinese media last April to expose the official truth on the Sars virus was in fact a lie.

"I felt I had to reveal what was happening," he has said, "not just to save China, but to save the world."

The Chinese health minister had called a press conference reassuring the public that Beijing had only a handful of cases of the deadly respiratory disease, but Dr Jiang, then working in a military hospital, said he knew of more than 100 people who were suffering from Sars in military hospitals alone.

His revelation led to the dismissal of the health minister and the mayor of Beijing, and an admittance by the authorities that the virus was widespread.

Political dynamite

He was dubbed "the honest doctor" by the weekly magazine Caijing.

The Chinese authorities are unlikely to concede Dr Jiang is right this time, if only because many of those currently in power did benefit politically from the Tiananmen Square crackdown.

But Ding Zilin, the mother of a 17-year-old victim of the Tiananmen protests, has said Dr Jiang's actions are needed to jog China's collective memory.

"The Chinese people forget their suffering too easily, which means that they are thrown into disaster time and again," she said. "We need more people like Doctor Jiang."